The present invention relates to a thermally conductive sheet having excellent thermal conductivity. Specifically, it relates to a thermally conductive sheet having both of excellent thermal conductivity and electric insulation performance, which effectively dissipates heat generated in various semiconductor devices, power supply, light source, parts or the like to outside the electrical apparatus.
To dissipate heat generated by various electronic parts in electrical apparatus (for example, exothermic electronic parts such as transistor and thyristor), heat-radiating members such as heat sink are conventionally attached to the exothermic parts through a thermally conductive sheet having good thermal conductivity. This thermally conductive sheet serves to reduce contact thermal resistance between the exothermic electronic parts and the heat-radiating member to conduct efficiently the heat generated in the exothermic electronic parts to the heat-radiating member.
As a thermally conductive sheet having good thermal conductivity, a sheet is generally known in which thermally conductive fillers having large thermal conductivity, such as metals, ceramics, and carbon fibers, are dispersed in an organic matrix. Such a kind of thermally conductive sheet is usually formed by dispersing thermally conductive fillers in an organic matrix, then molding the resultant mixed composition into a sheet by well-known molding techniques, such as a press molding, injection molding, extrusion molding, calendering, roll forming, and doctor blade molding.
In recent years, in high-performance electronic apparatuses, such as notebook type personal computers and mobile phones, the amount of heat generation in various electronic parts increases as electronic apparatuses are provided with higher integration, higher speeds, smaller sizes, and less weights. Thus, the need is growing more and more for thermally conductive sheet having still higher thermal conductivity. To meet such a need, various thermally conductive sheets are proposed in which boron nitrides (BN) with excellent thermal conductivity and electric insulation performance, especially hexagonal boron nitride (h-BN) having high thermal conductivity after BeO in ceramic materials, is dispersed in an organic matrix as thermally conductive filler.
It is known that hexagonal boron nitride have a crystal structure of a lamellar structure stacked with hexagonal network planes similar to graphite, and its particle is in a form of a flake due to its manufacturing process. This cause hexagonal boron nitride to have anisotropy in thermal conductivity. That is, hexagonal boron nitride powder has a feature that a thermal conductivity in a direction along the plane of a flaky particle is dozens times higher than a thermal conductivity in the thickness direction.
Therefore, it is expected to obtain a thermally conductive sheet whose thermal conductivity in the sheet thickness direction is greatly improved by dispersing the hexagonal boron nitride powder into an organic matrix so that planes of flaky particles of the powder are oriented along the sheet thickness direction.
However, as shown in FIG. 2, in the thermal conductive sheet molded by commonly well-known molding methods above mentioned, the planes of flaky particles of hexagonal boron nitride powder 23 tends to oriented in an organic matrix 22 so as to be parallel relative to the sheet surface by pressure and flow when molding. As a result, a thermally conductive sheet 21 obtained in this way has an excellent thermal conductivity in a parallel direction relative to the sheet surface. Accordingly, such thermally conductive sheet has a disadvantage that it cannot fully demonstrate an excellent thermal conductivity of the hexagonal boron nitride in applications where a thickness direction of sheet serves as a heat conduction path.
For above applications, various methods for orienting the boron nitride powder dispersed into an organic matrix material so that planes of the flaky particles are parallel relative to a sheet thickness direction are proposed.
For example, following methods are disclosed: in Japanese Patent Laid-Open No. 62-154410, a method in which boron nitride powder is randomly oriented in a sheet by using ultrasonic shaking machine; in Japanese Patent Laid-Open No. 3-151658, methods in which a heat-radiating sheet is formed by slicing a extruded sheet with boron nitride oriented in a fixed direction or by pre-forming to orient the filler and molding the preformed sheet; in Japanese Patent Laid-Open No. 5-174623, a method in which boron nitride powder is oriented by gravity and electrostatic force, and in Japanese Patent Laid-Open No. 7-111300, a method in which particle thickness of boron nitride powder is specified to 1 xcexcm or more. Moreover, Japanese Patent Laid-Open No. 8-244094 describes a method in which a plurality of plastic bands where hexagonal boron nitride powder is oriented in a fixed direction by extruding are integrated into a sheet by means of a die lip; Japanese Patent Laid-Open No. 11-77795 describes a method to form a rubber sheet in which a plurality of belt-like sheets is extruded by using a die with slits and combined together, and then the combined material is sliced in vertical direction relative to its thickness; Japanese Patent Laid-Open No. 11-156914 also describes a method in which a plurality of belt-like moldings are extruded using a first and a second molds having cavernous block structure in two steps and then bundled together in order to improve the degree of desired orientation in boron nitride powder; and Japanese Patent Laid-Open No. 2000-108220 proposes a method in which two or more of non-cured cylindrical moldings oriented by extruding is bundled with gaps to be pores.
Since each of these methods, however, requires special equipment or complicated manufacturing process, they are disadvantageous in respect of productivity or cost, and also limit a degree of freedom for thickness of sheets in design.
On the other hand, Japanese Patent Laid-Open No. 11-60216 proposed a thermally conductive sheet whose thermal conductivity in sheet thickness direction is improved by blending boron nitride powder having a specific degree of aggregation into an organic matrix. According to this proposal, special equipment or complicated manufacturing process becomes unnecessary, and a thermally conductive sheet having thermal conductivity excellent in sheet thickness direction can be obtained simply and at low cost.
However, since boron nitride powder used above has low wettability to an organic matrix due to their large specific surface area, there was a disadvantage that such powder could not be loaded with high concentration into the organic matrix. High level loading of such boron nitride powder with low wettability into the organic matrix requires addition of solvent, and consequently additional processes, such as solvent elimination.
To deal with the disadvantages abovementioned, an object of the present invention is to provide a thermally conductive sheet that has an excellent thermal conductivity in sheet thickness direction, and that can be simply produced with high productivity at low cost.
The present invention provides a thermally conductive sheet comprising boron nitride powder as a first thermally conductive filler dispersed into an organic matrix, wherein the boron nitride powder is hexagonal boron nitride (h-BN), and comprises primary particles and secondary aggregates formed by aggregation of the primary particles, wherein at least part of the secondary aggregates is 50 xcexcm or more in size. The secondary aggregates of 50 xcexcm or more in size are preferably contained in the boron nitride powder with 1 to 20 percent by weight.
The present invention also covers a mixed composition comprising boron nitride powder as a first thermally conductive filler dispersed into an organic matrix, wherein the boron nitride powder is hexagonal boron nitride (h-BN), and comprises primary particles and secondary aggregates formed by aggregation of the primary particles, wherein at least part of the secondary aggregates are 50 xcexcm or more in size.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.